Aerobic Granular Sludge Processes
Granular biomass processes for wastewater treatment, were originally limited to anaerobic treatment such as the upflow anaerobic sludge blanket process (UASB), which relies on granular biomass established with a specialty-built upflow reactor to allow symbiotic growth of several different classes of microorganisms, including fermentative, acidogenic, acetogenic, and methogenic. Additional development led to aerobic granular sludge processes being reported in the literature as early as 1997 (Morgenroth E, Sherden T, van Loosdrecht M C M, Heijnen J J, Wilderer P A. Aerobic granular sludge in a sequencing batch reactor. Water Res 1997; 31:3191-4). These processes are characterized by biomass with a higher density and particle size than flocculent biomass, and to date, have all been accomplished in specialty built reactors, primarily associated with sequencing batch reactors and upflow column reactors or reactors providing high shear conditions. The granular biomass has a particle size ranging from about 0.1-5 mm and a sludge volume index (SVI30 min) less than 35-50 mL/g and a SVI5 min that is similar to the SVI30 min. Similarly, aerobic granules have a settling velocity >10 m/h as opposed to approximately 1 m/h for flocculent biomass. The improved settleability of granular sludge over flocculent biomass is one of the important benefits of the method and apparatus according to the instant disclosure.
One key advantage of aerobic granular sludge is that it can create a niche condition within a granule for any condition that may be needed in separated physical tank volumes. Flocs and conventional activated sludges are subject to diffusion resistance (Shaw et al.), and aerobic granular sludge can take advantage of relative diffusion resistance inside and outside of a granule to develop and grow different populations simultaneously rather than to promote those conditions within physical tank configurations. A claimed benefit of aerobic granular sludge is that the size of the granule results in substrate and electron acceptor gradients within the granule allowing the accumulation of polyphosphorous accumulating organisms (PAO), glycogen accumulating organisms (GAO), anaerobic ammonia oxidizing bacteria (anammox), and denitrifying heterotrophic bacteria near the center of the granule, while aerobic organisms accumulate near the outside of the granule, including nitrifying bacteria and aerobic heterotrophs.
An example of this approach is further explained below for phosphorus removal.
Phosphorous Removal
Phosphorus removal from wastewater is typically achieved through either chemical precipitation using iron or aluminum salts or through the application of an anaerobic selector to allow the accumulation of polyphosphate accumulating organisms which provide biological phosphorus removal. Both of these approaches have disadvantages compared to the processes and systems disclosed herein, whereby stable and reliable phosphorus and nitrogen removal can be achieved without the need for a formal anaerobic selector and without chemical precipitation.
Chemical Phosphorus Removal
Chemicals used for the removal of phosphorus through the formation of precipitates typically include the sulfate or chloride salts of aluminum, ferric iron, and ferrous iron. These chemicals can be added ahead of primary clarification, into the biological process itself (typically activated sludge), or into the primary clarification process, ahead of a tertiary clarification or filtration process. The precipitated phosphorus is then removed from the wastewater flow with the solids stream leaving the primary clarifier, with the waste biomass, with tertiary clarifier solids, or with filter backwash waste, respectively. Problems with this approach include the need to purchase chemicals, the consumption of alkalinity as a result of adding these chemicals potentially requiring the addition of alkalinity and purchase or more chemical, the generation of additional sludge requiring further treatment and disposal.
Biological Phosphorus Removal
Biological phosphorus removal (bioP) is well known in the field of wastewater treatment and has the advantages over chemical phosphorus removal of decreased chemical costs, alkalinity demand, and sludge production but suffers from the need for a defined and formal anaerobic zone or period without contamination of dissolved oxygen or oxidized forms of nitrogen (nitrite and nitrate), proper wastewater characteristics in terms of volatile fatty acid (VFA) concentration, and often poor process reliability and upsets. BioP is generally accomplished by the accumulation of phosphate accumulating organisms (PAOs), which store phosphorus as polyphosphate (poly-P) as a source of energy. Under anaerobic conditions, PAOs cleave phosphate groups from poly-P, releasing phosphate to the bulk liquid, and from this obtain energy required to take up VFA. The VFA is stored as an intracellular macromolecule such as polyhydroxybutyrate (PHB). Reducing equivalents are also obtained by PAOs from the degradation of glycogen under anaerobic conditions. Under aerobic conditions, PAO take up phosphate to reform the intracellular poly-P pool and degrade the stored PHB for growth and energy through normal catabolic and anabolic pathways. Glycogen is also reformed under aerobic conditions. The process of bioP is therefore accomplished by subjecting typically flocculent biomass to alternative anaerobic and aerobic conditions according to the schematic shown in FIG. 1, which represents a process known commonly as A2/O or Phoredox and which is capable of both bioP, nitrification, and denitrification. A requirement for this process is a formal anaerobic selector zone with adequate degradable organic material in the form of acetate, or more generally VFA.
With a proper ratio of VFA to phosphorus, PAOs are able to take up all of the phosphate released in the anaerobic zone and additional phosphate present in the wastewater, achieving a net removal of phosphate through biomass wasting. One challenge associated with the A2/O process as shown in FIG. 1 is that nitrate present in the return activated sludge (RAS) stream can enter the anaerobic zone, and this is well known to disrupt the bioP process. Another aspect of relevance to this disclosure is the ability of some PAO to denitrify (dPAO), whereby nitrate can serve as the electron acceptor instead of oxygen, allowing phosphorus uptake in the anoxic zone. While phosphate uptake by dPAO is known to be significantly slower than under aerobic conditions, the important benefit of maximizing this metabolism is efficiency of use of the same pool of organic carbon in the incoming wastewater for both bioP and nitrogen removal. Limitations of the bioP process include poor reliability in terms of consistent compliance with low effluent total phosphorus limits and strict reliance on the availability of an appropriate quantity of VFA in the incoming wastewater.
Reactor Configurations for Aerobic Granular Biomass
One such reactor is disclosed in U.S. Pat. No. 5,985,150, which appears to be assigned to Biothane Systems International B.V. In this patent, there is disclosed an airlift reactor providing enhanced shear in which granular sludge is used to treat wastewater. The granular sludge is carried with the upwardly flowing gas into a settling region that applies a relative overflow rate to help in the granular biomass selection process, with return of the underflow to the aerated section of the reactor.
U.S. Pat. No. 6,566,119 B1 describes an aerobic granular sludge process accomplished in a sequencing batch reactor operated with very short settling and decant periods to select for granular biomass with excellent settling properties.
U.S. Pat. No. 6,793,822 B2 describes an aerobic granular sludge process for which the granules may be created in a sequencing batch reactor by the methods of, e.g., U.S. Pat. No. 6,566,119 B1, and enhanced by the shear provided by a high superficial gas velocity of the diffused bubble aeration system.
US Patent Application Publication No. US 2006/0032815 A1 discloses an aerobic granular sludge process that appears to have been commercialized as the full-scale Nereda® process by Royal Haskoning DHV. The features of this sequencing batch reactor aerobic granular sludge process involve wasting of the fraction of slowly settling biomass from the process itself and feeding wastewater in an upflow manner through a stagnant and anaerobic layer of settled granules. This allows VFA to diffuse into the granule where PAO and GAO are established. The process is then aerated to achieve simultaneous nitrification-denitrification and denitrification by dPAO.
PCT application publication no. WO 2013/151434 A1, which appears to be assigned to Royal Haskoning DHV, discloses the transfer of waste biomass from a granular sludge process, such as that disclosed in, e.g., US 2006/0032815 A1, into a flocculent biomass process, such as the conventional activated sludge process so as to gain the benefits in terms of settleability and nitrogen and phosphorus removal in the activated sludge process.
While similar to US 2006/0032815 A1, PCT application WO2012/175489 A1 appears to improve on this process by fluidizing the bed of granules under anaerobic conditions and provides further mixing during the anaerobic period prior to aeration.
PCT application publication no. WO 2008/141413 A1 describes a sequencing batch reactor operated to promote granulation and phosphorus and nitrogen removal, with the added feature that following the anaerobic phosphorus release period, a portion of the reactor contents may be discharged from the reactor to conduct chemical precipitation of phosphate.
External Gravimetric Selection for Anammox Granule Formation and Accumulation
US Patent Application Publication No. US 2011/0198284 A1 describes the application of an external gravimetric selector for the formation and accumulation of anaerobic ammonia oxidizing bacteria (anammox) containing granules in the process. In this disclosure, it appears that the selection device could be a hydrocyclone, a centrifuge, or a high overflow rate gravity settling device. This disclosure appears to demonstrate the validity of using an external gravimetric settling device to select for anammox biomass in either a mainstream or sidestream process.
External Gravimetric Selection for Settleability Improvement
US Patent Application Publication No. US 2014/0144836 A1 describes the use of an external gravimetric selector for the selection of granular biomass in a suspended growth biological wastewater treatment process for the benefit of improved biomass settleability. In this disclosure, it appears that the selection device could be a hydrocyclone, a centrifuge, or a high overflow rate gravity settling device. This disclosure appears to demonstrate the validity of using an external gravimetric settling device to select for biomass with superior settling characteristics.
Nitrogen and Phosphorus Removal by Struvite Precipitation
Struvite is often formed during anaerobic digestion and in sludge piping, dewatering equipment, and sludge dewatering liquor piping due to high levels of phosphate and ammonia and limiting but sufficient magnesium levels. Often low pH can also limit struvite precipitation. Struvite precipitation and recovery can be used to achieve nitrogen and phosphorus removal, and this has been done using an upflow fluidized reactor with magnesium and alkalinity addition, as disclosed in, e.g., U.S. Pat. No. 7,622,047 B2. Furthermore, removal of excess magnesium and phosphorus from anaerobically digested sludge, either through the addition of alkalinity or by aeration to strip excess carbon dioxide and increase pH, and the subsequent recovery of the precipitated struvite, can decrease the risk and extent of unintentional downstream struvite scaling and the associated maintenance requirements.
The selection device or method could also be a filter or a screen that selects based on size instead of gravimetric selection